Mostly open and nothing left in the instrument (8)
I believe the answer is:
mandolin
'the instrument' is the definition.
(mandolin is a kind of instrument)
'mostly open and nothing left in' is the wordplay.
'open' indicates taking the first letters (I've seen 'opens' mean this).
'nothing' becomes 'O' (looks like zero - 0).
'left' becomes 'l' (common abbreviation).
The first letter of 'mostly' is 'm'.
'm'+'and'+'o'+'l'+'in'='MANDOLIN'
(Other definitions for mandolin that I've seen before include "vegetable slicer" , "Round-backed musical instrument" , "Stringed instrument, usually played with a plectrum" , "Small guitar-like instrument" , "Instrument resembling a lute" .)